1. Field of the Invention
The present invention relates to a control device for an internal combustion engine which has a function of calculating a target engine intake air flow quantity in the vicinity of a throttle valve such that a target cylinder intake air flow quantity is obtained with good responsiveness so as to control a throttle opening degree based on the target engine intake air flow quantity.
2. Description of the Related Art
In recent years, there has been proposed a control device for an internal combustion engine which uses a physical quantity acting directly on the control of a vehicle, namely, an output shaft torque of the internal combustion engine (engine) as a required value of a driving force from a driver or the vehicle side, and determines engine control amounts, namely, an air amount, a fuel amount, and an ignition timing with the output shaft torque adopted as an engine output target value so as to obtain good running performance.
It is generally known that an air amount has the greatest influence on an output shaft torque of an engine among engine control amounts. Therefore, there has also been proposed a control device for an internal combustion engine which controls an air amount with high accuracy.
In general, an intake system of an internal combustion engine is physically modeled as a first-order lag system (first-order lag filter). During steady operation, an intake air amount of the internal combustion engine (actual engine intake air flow quantity) and an amount of air sucked by cylinders of the internal combustion engine (actual cylinder intake air flow quantity) are considered to substantially coincide with each other. During transitional operation, however, the intake air amount of the internal combustion engine and the amount of air sucked by cylinders of the internal combustion engine do not coincide with each other (e.g., see JP 05-38143 A).
A conventional fuel control device for an internal combustion engine disclosed in JP 05-38143 A is equipped with an AN detection unit for detecting an intake air amount of the internal combustion engine by means of an intake air amount sensor disposed upstream of a throttle valve so as to detect an output as a result of this detection within an interval of a predetermined crank angle, an AN calculation unit for calculating an amount of air sucked by the internal combustion engine, and a control unit for controlling an amount of fuel supplied to the internal combustion engine based on the air amount calculated by the AN calculation unit. The AN calculation unit calculates the amount of air sucked by the internal combustion engine, using an amount of air sucked by the internal combustion engine during a preceding stroke and a first-order lag filter.
However, the conventional device disclosed in JP 05-38143 A does not take into account a volumetric efficiency equivalent value of air sucked into cylinders from an intake pipe, which includes the influences of intake valves and exhaust valves of the engine.
Therefore, in an engine having, for example, a mechanism for variably controlling intake valves and exhaust valves, there is a problem in that a great error occurs between a flow quantity of intake air actually sucked into the cylinders and an air amount calculated by the AN calculation unit in a certain operation range during transitional operation.
In order to solve the above-mentioned problem, another conventional control device for an engine is equipped with a fresh air amount detecting unit for outputting a detected value of an amount of fresh air passing through an intake passage (actual engine intake air flow quantity), an efficiency calculating unit for calculating a volumetric efficiency equivalent value, and a fresh air amount estimating unit for estimating a predicted value of an amount of fresh air flowing into combustion chambers (actual cylinder intake air flow quantity). The fresh air amount estimating unit estimates the predicted value of the amount of fresh air based on the detected value of the amount of fresh air and a change in the volumetric efficiency equivalent value (e.g., see JP 2005-54657 A).
However, the conventional device disclosed in JP 2005-54657 A does not take into account a method of controlling response characteristics during transitional operation.
Therefore, a long response delay of air for a change in throttle opening degree is observed in, for example, a low-load range. Thus, there is a problem in that the rotational speed of the engine temporarily drops due to an increased discrepancy between a target engine intake air flow quantity corresponding to a target torque of the engine and the detected value of the amount of fresh air passing through the intake passage (actual engine intake air flow quantity).
In order to solve the above-mentioned problem, another conventional control device for an internal combustion engine calculates an amount of air sucked into cylinders of the engine (actual cylinder intake air flow quantity) and a target throttle opening degree, respectively, using a normative model realizable in an entire range for a target intake air amount obtained from a target torque through conversion (target engine intake air flow quantity) and an inverse model of a response model of an intake air amount for a change in target throttle opening degree, thereby performing control to ensure substantially constant responsiveness of an actually measured torque or an estimated torque for the target torque.
This control device also corrects the target throttle opening degree in a feedback manner such that a difference between an output of the normative model and an amount of air actually sucked by the engine decreases, thereby causing the output of the normative model and the amount of air actually sucked by the engine to coincide with each other (e.g., see JP 2006-70701 A).
The conventional control device for the internal combustion engine disclosed in JP 2006-70701 A entails complicated arithmetic expressions and hence a large number of constants to be matched. Therefore, there is a problem in that an increase in the number of man-hours for matching, an increase in the number of man-hours for check and assessment, and an extension of calculation time are caused.